Resistive devices



April 6, 1968 .1. A. CARTWRIGHT 3,378,804

RESISTIVE DEVICES Filed Jan. 26, 1966 2 Sheets-Shet 1 April 16, 1968 J. A. CARTWRIGHT RES ISTIVE DEVICES 2 Sheets-Sheet 2 Filed Jan. 26, 1966 United States Patent 3,378,804 RESISTIVE DEVICES John Antony Cartwright, Links Estate, Northampton, England, assignor to Painton & Company Limited, Kingsthorpe, Northampton, England, a British com- Filed Jan. 26, 1966, Ser. No. 523,186 Claims priority, application Great Britain, Mar. 30, 1965, 13,490/ 65 7 Claims. (Cl. 338-174) ABSTRACT OF THE DISCLOSURE This application discloses a manually adjustable resistance device having connecting elements between which the resistance is varied by relative rotation between an arcuate resistive element and pick-up means engaged therewith. A follower wheel carrying said pick-up means is rotated by speed reduction gearing from a manually regulated thumbwheel. Both the follower wheel and the thumbwheel have a portion of their rims projecting from a casing housing said device and are so arranged that a given rotational movement of the thumbwheel produces a substantially parallel lesser movement of the carrier.

This invention relates to resistive devices and, more specifically, is concerned with a manually adjustable resistive device such as a potentiometer or variable resistance. The device is provided with tags for connecting into an electrical circuit and is adjustable to vary the electrical resistance between the tags. Throughout this specification and claims the expression tags is to be understood as meaning electrical elements for enabling external electrical connections to be made to electrically operative parts of the device. Such tags may, for example, take the form of projecting wires, strips or terminals, or any other form of electrical connecting element.

The well-known form of manually adjustable resistive device has an arcuately arranged resistance element and a carrier which is manually rotatable about the axis of the resistive element and carries a wiper which engages therewith. A tag on the device is electrically connected to the Wiper, for example by means of a slip ring, and at least one other tag is provided which is connected to the resistive element, usually at one end. The carrier is commonly constructed integrally with a shaft carrying a knob at its end which is rotatable to vary the posiion of the carrier and thus the electrical resistance between the tags.

In the electronics filed there is an increasing demand for electrical components of progressively smaller size which are capable of providing the same performance as conventional components of larger size. Although the conventional resistive device having an arcuate resistive element as described above can be made in a small size it cannot be controlled by hand with the desired degree of accuracy.

This has led to the development of other constructions of adjustable resistive devices. One such construction is the lead screw potentiometer. This comprises a casing containing a linear resistive element which extends parallel to a lead screw having a slotted head projecling from one end of the casing. The lead screw is rotated by means of a screwdriver applied to its slotted head and has part of a wiper element wrapped around it. The wiper bridges between the resistive element and a contact strip parallel to it, and engages threads on the lead screw. The wiper engages guide surfaces which prevent it from rotating when the lead screw is turned so that rotational movement of the lead screw is translated into linear movement 3,378,804 Patented Apr. 16, 1968 of the wiper. This results in a small movement of the wiper being obtained by a large movement of the lead screw head and thus accurate control of the wiper position is obtained. However this advantage is otf-set by the disadvantage that the straight resistive element causes the-potentiometer tobe rather long if a fine control of resistance is to be obtained over a large range of resistance values.

This has led to the development of a further construction of resistance device, known as the worm screw potentiometer, where fine control is obtained without having to extend the casing to accommodate a long resistive element.

In the worm screw potentiometer the resistive element is circular and is engaged by a rotatable .wiper carrier by a suitable carrier centrally mounted inside the resistive element. The carrier is provided with a peripherally toothed rim which is engaged by a tangentially arranged worm shaft. A slotted head is provided on the worm shaft outside the casing and enables the worm to be turned by means of a screwdriver. The worm shaft meshes with the toothed rim of the carrier so that the rotational position of the wiper around the resistive element can be finely controlled by the slotted head.

Although the worm screw potentiometer can, for given external dimensions, provide as fine a control of wiper position as the lead screw potentiometer over a greater range of resistance, it suffers from a disadvantage present also in the lead screw potentiometer. In both forms of potentiometer adjustment is effected by a screwdriver and the small size of the slotted head makes adjustment difficult, particularly when the potentiometer is mounted in an inaccessible position, unless a screwdriver of exactly the right shape and size is used. Adjustment by hand is dii'licult and sometimes impossible and it is also not possible to determine visually, with any degree of accuracy, the precise rotational position of the slotted head. This makes it difiicult to access the value of resistance introduced into an electrical circuit by the potentiometer, without actually measuring the resistance.

An object of this invention is the provision of an improved resistive device which is manually adjustable, with a relatively high degree of accuracy.

A manually adjustable resistive device having connection tags between which the resistance is varied by relative rotation between an arcuate resistive element and pick-up means engaged therewith, has, in accordance with one aspect of the invention, the necessary rotation produced by a rotatable carrier driven through speed reduction gearing from a thumbwheel whose rim moves along a path which is substantially parallel to the path of movement of the carrier. The thumbwheel is arranged so that a portion of its marginal edge is located at a position at which it is readily accessible for adjustment by the thumb of the user. The use of gearing in the device enables a more accurate control of the rotational movement of the carrier than has hitherto been possible where the movement has been controlled directly by the hand of the user without intermediate gearing.

In accordance with a second aspect of this invention a manually adjustable electrical resistive device includes an insulative casing provided externally with tags electrically connected inside the casing to an arcuate electrical resistive element, an electrical pick-up mounted on a rotatable carrier and engaging the resistive element whereby rotation of the carrier varies the position of the pick-up on the resistive element, means electrically connecting the pick-up to one of the tags, the other tag be-- ing electrically connected to the resistive element, a rotatable thumbwheel having its rim projecting from the casing at a manually accessible position, and reduction gearing mechanically interconnecting the thumbwheel and the carrier whereby a given rotational movement of the thumbwheel rim produces a lesser substantially parallel movement of the carrier. The parallel movement of the carrier may be in the same direction or in the opposite direction to the movement of the thumbwheel.

In accordance with a third aspect of this invention a manually adjustable electrical resistive device includes an insulative casing provided externally with tags electrically connected inside the casing to an arcuate electrical resistive element, a rotatable carrier wheel having a portion of its rim provided with calibrations and projecting from the casing, an electrical pick-up mounted on the carrier wheel and engaging the resistive element whereby rotation of the carrier varies the position of the pickup on the resistive element, means electrically connecting the pick-up to one of the tags, the other tag being electrically connected to the resistive element, a rotatable thumbwheel arranged beside the carrier wheel and also having a portion of its rim projecting from the casing at a manually accessible position, and reduction gearing mechanically interconnecting the thumbwheel and the carrier whereby a given rotational movement of the thumbwheel rim produces a lesser substantially parallel movement of the carrier.

The invention is capable of providing a very compact construction of resistive device, particularly when the thumbwheel and carrier are arranged coaxially with the resistive element. Suitably the carrier comprises a follower wheel having a flanged rim provided externally with markings for indicating the accurate position of the carrier.

Although the resistive element may be mounted on the carrier it is preferred to have the resistive element fixed and the carrier to be provided with the pick-up means, such as contact strips or wipers, which are carri ed around the resistive element by rotation of the carrrer.

The invention may be utilised to provide a resistive potentiometer or merely a variable resistance. It has been found in practice that a speed reduction of 10 to 1 between the thumbwheel and the carrier is to be preferred and, for this purpose, a single gear wheel may be used which may be arranged to have a toothed outermost edge meshing with a gear formed on a shaft on the thumbwheel, and a toothed shank meshing with teeth formed on the periphery of a toothed hub on the carrier. A slipping clutch may also be provided.

The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which:

FIGURE 1 shows, in elevation, a manually adjustable electrical resistive device constructed as a potentiometer;

FIGURE 2 is an end view of FIGURE 1;

FIGURE 3 is an exploded perspective view of the potentiometer from one side;

FIGURE 4 is an exploded perspective view of the potentiometer from the other side;

FIGURE 5 shows, in plan, a gear-wheel used in the potentiometer; and,

FIGURE 6 is a side view of FIGURE 5.

Referring to FIGURES l and 2 the potentiometer is mounted inside a casing 1 which is 0.65 inch long, 0.3 inch thick and 0.63 inch high. The casing is formed by two half-casings or covers 2, 3 which interfit along a line parallel to the length of the casing and which are held together by two bolts 4 having nuts 5. Each cover has a lower portion 6 which is rectangular when viewed from the side and the ends, and an upper portion formed by a semicircular flange 7 continuous with the side wall of the casing. The semi-circular space formed between the two spaced flanges 7 contains the upper halves of a thumbwheel 8 and a coaxially arranged follower wheel or carrier 10 of the same diameter. The follower wheel and thumbwheel are provided with flanged rims 12, 13 which, at their edges, locate in semi-circular grooves 11 formed in the two covers respectively. The carrier rim 12 bears numbers on its outside surface and the thumbwheel rim 13 is knurled and projects slightly beyond the carrier rim to enable the thumbwheel 8 to be readily rotated by the thumb of the user.

The cover 2 adjacent the thumbwheel will hereinafter be referred to, for convenience, as the thumbwheel cover to distinguish it from the other cover 3. The inside wall of the thumbwheel cover 2 is provided with two annular grooves 14, 15 both of which are rectangular in crosssection. It is also provided with a reference index 19 formed by an arrow scribed onto its flange 7.

In wardly of its rim 13 the thumbwheel 8 comprises a plastics disc having one flat surface which engages the thumbwheel cover 2 and a coaxial drive shaft 16 which projects from its center towards the cover 3 The root portion 17 of the drive shaft is externally toothed while the terminal portion 18 is of smooth cylindrical shape and extends into a well 20 formed in the central portion of the follower wheel 10 and which provides a journal for it.

The follower wheel 10 is integrally formed with an externally toothed hub 21 on its side opposite the rim 12. The well 20 is formed in the center of the hub 21 and an arouate segment 22 of the hub periphery is free of teeth. Radially outwardly of the segment 22 and disposed immediatcly behind the rim 12 is a projection 23 providing with its opposite ends two spaced abutments.

The thumbwheel 8 and follower wheel 10 are mechanically interconnected by a gear wheel 24 mounted for rotation in the lower part of the casing 1 about an axis parallel to the axis of the drive shaft. The gear wheel 24 is shown in detail in FIGURES 5 and 6 and has an outer toothed rim 25 which meshes with the externally toothed root 17 of the shaft 16, and an externally toothed shank 26 which meshes with the teeth formed on the hub 21 of the follower wheel. The gearwheel is integrally formed with stub shafts 27 which are freely rotatable in bearings provided by notches on a partition plate 28. Plate 28 has a generally rectangular lower half which is clamped at its edge between the covers 2 and 3, and a semi-circular upper half having its edge disposed between the exposed rims 12 and 13 of the follower wheel 10 and the thumbwheel 8. A circular opening 30 is formed in the center of the partition plate 28 and surrounds the toothed hub 21 of the follower wheel. An arcuate bracket 31 of Z-shape is spot welded to the lower half of the partition plate 28 around the central opening 30 and its free arm 32 extends in curved fashion around the marginal edge of lower half of the opening 30 while being spaced therefrom as shown in FIGURE 4. A notch 33 is formed in the arm 32 and a similar registering notch 34 is formed in the opening 30. The spacing between the arm 32 and the plate 28 is such that the gear wheel 24 can be mounted between them with its two stub shafts 27 resting in the notches 33, 34 respectively.

A clutch spring 35 is carried by the partition plate 28 on its side opposite the bracket 31. The spring comprises a length of thin Phosphor bronze wire having its intermediate portion forming a finger 36 which locates in a pressed out portion 37 of the plate 28 as shown in FIG- URE 3, and which is brazed in position. The two end portions of the wire are bent to form two C-shaped spring arms 38, 38' which encircle opposite sides of the hub 21 and are normally spaced from it, the terminal parts of the arms, 38, 38' being bent away from the plane of the plate 28 to provide two tabs 40 lying in the path of movement of the projetcion 23 formed on the follower wheel 10.

The follower wheel 10 is provided inwardly of its rim 12 with two thin short studs 41 which project towards the cover 3 from diametrically opposite positions. An electrically conductive contact disc 43 made of Phosphor bronze lies against the studded face of the follower wheel 10 and is provided with two holes 44 through which the studs 41 extend. The studs serve to hold the contact disc 43 in position and the disc 43 is a press fit over them. The ends of the studs are formed into beads by heat and pressure to anchor the contact disc 43 firmly to the follower wheel 10. The contact disc 43 is provided with pick-up means formed by three arcuately extending wiper contacts or pick-up strips 45, 46 and 47 which are pressed out of the disc 43 and extend obliquely to its plane towards the cover 3. Two of the strips, 45 and 46, are disposed on the same circle but extend in opposite directions respectively, whereas the third contact strip 47 is of larger size and is disposed adjacent the outer edge of the contact disc 43 so that it is spaced inwardly of the rim and lies opposite a part-circular, resistance element 48 held in a circular channel 50 formed in the inside wall of the cover 7 as shown in FIGURE 4. The free ends of the pick-up contact strips are hooked to provide bearing surfaces which are gold plated.

The resistive element 48 is formed by winding resistive wire in closely-spaced turns on a thick insulating plastic sleeve formed on a copper rod which is bent into the shape of a broken ring. Attached to each end of the resistive element 48 is a conductive wire which extends through the plastic material from which the cover 3 is moulded and projects out of its base to form a tag 52. The two tags 52 connected respectively to opposite ends of the resistive element are aligned with one another and with a central tag 53 which also extends through the moulding forming the cover 3 and has brazed to its inner end a flat take-off ring 54 which seats in an inner portion 39 of an annular channel 49 of stepped cross-section formed in the cover at a position such that the ring is engaged by the two oppositely directed contact strips 45-46 of the contact disc 28. The three tags are moulded in situ when the cover 3 is moulded.

The cover 3 is provided with holes which register with corresponding holes in the cover 2 to enable the bolts 4 to be passed through them when the two covers are fitted together. The cover 3 is also provided with two steel guide pins 57 which assist assembly of the potentiometer by locating in complementary holes 58 formed in the contact disc 28 and the other cover 2.

The thumbwheel 8, gear wheel 24 and follower wheel 10 are made of nylon which is a self-lubricating material. The gearing is so arranged that the thumbwheel 8 re volves ten times to produce 300 of revolution of the follower wheel 10. Thus the precise position of the contact strip 47 on the resistive element 48 can be controlled with a hitherto unobtainable degree of accuracy. Moreover, the markings provided enable the precise position of the contact strip 47 on the element 48 to be identified.

The potentiometer is assembled as follows.

The resistive element 48 and the ring 54 are placed in their operating positions in the cover 3 and cemented in position. The contact disc 43 is pressed into position on the follower wheel so that the two heat-deformable studs 41 project through the holes 44 and the projecting ends of the studs are then deformed by heat and pressure into heads which secure the contact disc 43 to the follower wheel. Next the follower wheel is mounted on the cover so that the edge of its rim 12 seats in the groove 11 in the cover 3 and the contact strips 45, 46 and 47 engage the ring 54 and the resistive element 48.

After assembly of the bracket 31 and the clutch spring 35 on the partition plate 28, the gearwheel 24 is cradled between the two notches 33, 32. The follower wheel is then rotated until the toothless segment 22 of the hub 21 comes opposite the position of the teeth on the gearwheel shank 26, and the plate 28 is then eased gently into position so that the guide pins 57 pass through the holes 58 in the plate 28 and the hub 21 passes through the aperture 30. To enable the shank teeth to overlap the hub 21 it is necessary to rotate the follower wheel 10 against the stop provided by the engagement of the projection 23 with one of the tabs 40 of the spring arms 38. The associated arm 38 yields resiliently to allow the untoothed segment 22 of the hub to come opposite the shank of the gear wheel which then drops into its operating position. As soon as the rotational force on the follower wheel 10 is removed, it is immediately rotated by the resiliently flexed arm 38 through a small angle suflicient to cause the toothed portion of the hub 21 to come opposite and mesh with the teeth on the gear wheel shank.

The thumbwheel 8 is then mounted in position so that the terminal position of the shaft 16 fits into the hub well 20 which provides a journal for it, and the toothed root 17 of the shaft 16 meshes with the toothed edge 25 of the gearwheel 24. To complete the assembly the cover 2 is fitted over the thumbwheel with the assistance of the guide pins 57 locating in the holes 58, and the bolts 4 are passed through the bolt holes in the covers 2, 3 and the nuts 5 are screwed onto their projecting ends.

To use the potentiometer the thumbwheel 8 is rotated and its movement is transmitted through the gearwheel 24 to the follower wheel 10 so that the contact strip 47 is moved a distance around the resistance element 48 equal to of the arc of movement of the thumbwheel 8. The two contact strips 45, 46 engaging the flat ring 54 provide parallel paths from the contact strip 47 to the ring 54 so that the effect of contact resistance is negligible. Also, by virtue of the two contact strips 45, 46 engaging the take-off ring 54 extending in opposite directions, the electrical resistance measured at a particular place when the thumbwheel 8 is moved in one direction to a selected position is the same as the electrical resistance measured when the thumbwheel 8 is moved in the other direction to the same position.

When the contact strip 47 reaches the ends of its travel around the resistance element 48 a slipping clutch comes into operation so that further rotation of the thumbwheel in the same direction does not cause damage to the components of the potentiometer. It will be appreciated that the 10:1 gear reduction could easily cause damage to components of the potentiometer should simple abutments be used to limit rotational movement of the follower wheel.

The slipping clutch operates as follows:

When the follower wheel approaches the end of its rotational movement in either direction, one end-face of the projection 23 engages one of the outwardly turned tabs 40 formed on the spring arms 38 and causes the spring arm to flex resiliently into engagement with the toothed periphery of the hub 21. When the'spring arm 38 contacts the hub 22 the tab 40 prevents further rotation of the follower wheel in the same direction and simultaneously one end of the untoothed segment 22 of the hub 21 comes opposite the toothed shank of the gearwheel 24. This further rotation of the thumbwheel 8 in the same direction rotates the gearwheel 24 but the drive is not transmitted to the follower wheel as the toothed shank 26 is no longer meshing with the toothed portion of the hub 21. As soon as the force turnin the thumbwheel 8 is removed the resiliently flexed spring arm 38 bows back out of engagement with the hub 22 and its outwardly-turned tab 40, acting on the projection 23, causes the follower wheel to rotate through a small angle in the reverse direction such that the toothed portion of the hub 21 is once again brought opposite and meshes with the toothed shank of the gearwheel.

From the above description it will be appreciated that the invention results in a very compact potentiometer capable of being finely controlled with a high degree of accuracy.

The invention is not limited for use with potentiometers as it is also applicable to variable resistors. In this case one of the tags 52 is removed and the.slipping clutch is removed also. The exposed rim of the follower wheel is calibrated in terms of the resistance between the tags 52, 53 and the thumbwheel is provided with a vernier calibration. With such an arrangement a very accurate value of the resistance between the tags can be read from the device. It is necessary to avoid the use of a slipping clutch in such an arrangement to ensure that the vernier calibration and resistance calibrations remain in step with one another. Naturally the hub 21 in such an arrangement is toothed around its entire periphery and in many cases the use of stops is unnecessary as it is quite permissible for the follower wheel to be rotated an indefinite amount in either direction.

I claim:

1. A variable resistance device comprising a hollow casing, electrical connectors on said casing, an arcuate resistive element mounted inside said casing and electrically connected to one of said connectors, a rotatable thumbwheel mounted inside said casing, a rim portion of the said thumbwheel protruding from said casing, a rotatable carrier mounted inside the casing in coaxial relationship with said thumbwheel, speed reduction gearing mounted inside the casing and interconnecting said -thumb wheel and said carrier whereby a given movement of the thumbwheel produces a lesser movement of the carrier, stop means inside the casing, abutment means rotated with said carrier and positioned to engage said stop means to halt rotational movement of the carrier when at opposite ends of its rotational travel, slipping clutch means mounted inside the casing and co-operating with said stop means to interrupt drive transmission between the thumbwheel and carrier after engagement of said abutment means with said stop means, a contact member fixed inside said casing and connected to a second of said connectors, and contact carrier carried by the wiper and electrically connected to one another, one of said wipers engaging said contact member and the other of said wipers contacting the resistive element.

2. A variable resistance device as claimed in claim 1, in which a hub is provided on the side of the carrier adjacent the thumbwheel and is externally toothed over the major part of its periphery, the gearing comprises a gearwheel meshing with the thumbwheel and the toothed hub, and the slipping clutch comprises a pair of oppositely directed bow springs encircling the hub and having out wardly turned tabs providing said stops, engagement of the abutments with the spring stops coinciding with the untoothed peripheral part of the hub coming opposite the gearwheel whereby further rotation of the thumbwheel is not transmitted by the gearwheel to the carrier hub.

3. A variable resisance device as claimed in claim 1, in wich an apertured partition plate sandwiched between the thumbwheel and the carrier provides, with an attached bracket, a bearing for a gearwheel which has an outer ring of teeth meshing with a toothed root portion projecting from the center of the thumbwheel and an inner ring of teeth which meshes with an externally toothed huh projecting through the apertured partiton plate from the center of the carrier.

4. A variable resistance device comprising a hollow two-part casing, a pair of part-circular flanges in confronting relationship integral with respect to parts of said two-part casing and defining an elongated part-cylindrical opening therein, three electrical connectors projecting from one of said casing parts, an arcuate resistive element mounted inside said one casing part and electrically connected at opposite ends to two of said connectors respectively, a contact member mounted inwardly of said element on said one casing part and electrically connected to the third electrical connector, a carrier wheel rotatably mounted about an axis which forms the axis of symmetry of the resistive element and the part circular flanges, a contact plate fixed to said carrier wheel and carrying radially spaced wipers engaging the contact member and the resistive element, a rim portion of said carrier wheel projecting beyond the edge of one of said flanges in one side of the opening, a partition plate held at its edge between the two part casings, a rotatable thumbwheel coaxially arranged with respect to the carrier and mounted inside the casing on the opposite side of the partition plate to the carrier, said thumbwheel having a knurled rim portion filling the remainder of said opening and projecting therethrough slightly beyond the rim of the carrier, speed reduction gearing carried by said partition plate and in meshing relationship with arcuate toothed surfaces on the thumbwheel and carrier, stop means on said partition plate controlling operation of slipping clutch means on said partition plate, abutment means on said carrier engaged with said stop means at either end of travel of said carrier to operate the slipping clutch and interrupt the transmission of further drive in the same direction from the thumbwheel, and securing means holding the two partcasings together.

5. A variable resistance device according to claim 4, including calibration markings on the rim portion of said carrier in said opening and a fixed reference position with which the calibrations individually register to denote the position of the wiper engaged with the resistance element.

6. A variable resistance device according to claim 4, including two oppositely directed arched wipers projecting from one side of the contact plate radially inwardly of the resistance element wiper and engaging the same circular path on the contact member.

7. A variable resistance device according to claim 4, in which the gearing comprises a gear wheel disposed radially inwardly of the knurled rim of the thumbwheel and mounted in notch bearings formed in the partition plate and a bracket secured thereto, the gearwheel having an inner ring of teeth meshing with a toothed hub projecting from the center of the carrier through an aperture in the partition plate and formed in its center with a well, and an outer ring of teeth meshing with a toothed root portion of a shaft projecting from the center of the thumbwheel in-to a bearing provided by the hub well; the carrier, thumbwheel and gearwheel being made of nylon, and the slipping clutch means comprising a pair of oppositely directed bow springs fixed to the partition plate and encircling the hub, the free ends of the bow springs having outwardly turned tabs forming said stops means and a peripheral portion of the carrier hub being untoothed and coming opposite the gearwheel in opposite terminal positions of the carrier.

References Cited UNITED STATES PATENTS 2,620,418 12/1952 Knowles et a1, 338128 2,670,422 2/1954 Baum et al. 338-128 2,783,397 2/1957 Larson 338128 X 3,089,923 5/1963 Wright ZOO-11 X 3,129,382 4/1964 Scott 330-122 X ROBERT K. SCHAEFER, Primary Examiner.

H. HOHAUSER, Assistant Examiner. 

